The field of the invention is in the nondestructive testing art and more particularly in acoustic surface wave transducers for nondestructive surface examination.
Recent advances in the design of, and the electronics for, electromagnetic SAW transducers (SAW EMTs) have increased the dynamic range of these devices to the point where they are comparable in sensitivity to piezoelectric wedge transducers for NDE inspections of metals such as aluminum or magnetic steels. One remaining problem which, prior to this invention, has hindered the application of SAW EMTs to NDE problems is poor range resolution. This difficulty arises from the fact that the transduction efficiency is proportional to the square of the number of periods in the meander line. Practical devices usually have meander lines with 5 to 20 periods. When such a device is excited by a short pulse, the resulting output tone burst is NT long where N is the number of periods and T is the period. When an identical meander line is used to receive the generated tone burst, the burst is spread by an additional NT. Thus, it is not generally possible to generate and detect tone bursts of less than approximately 2NT duration with conventionally designed SAW EMTs. The problem is illustrated in FIG. 1 where the response of a transmitter-receiver pair of 16 period 5 MHz SAW EMTs to a 50 nsec pulse excitation is shown. The received tone burst in this case is 6 .mu.sec long (corresponding to a spatial extent of 1.8 cm). Such a pair of transducers would be virtually useless for a conventional ultrasonic NDE inspection if it were necessary to find flaws less than 1 cm apart or within 1 cm of an obstacle such as a bolt hole.
For additional background that may be helpful in understanding the present invention, attention is directed to the paper, IEEE Cat. No. 76 CH1120-55U presented at the 1976 Institute of Electrical and Electronic Engineers Ultrasonics Symposium, Sept. 29 to Oct. 1, 1976, entitled "Use of Pulse Compression Techniques to Improve the Range Resolution of Electromagnetic Surface Wave Transducers" by T. J. Moran. The following patents may be helpful in illustrating the prior art. U.S. Pat. Nos. 3,460,063 to patentees J. R. Houck et al, 3,550,435 to patentee W. Kaule, 3,786,672 to patentee M. R. Gaerttner, and 3,850,028 to patentees R. B. Thompson et al.